1. Field of the Invention
The invention relates to a gas-cooled electric machine as described in the preamble of the first claim.
A gas-cooled electric machine having these features is disclosed, for example, in DE-43 32 304 A1.
2. Discussion of Background
The principle of so-called indirect cooling is to dissipate the losses produced in the stator winding bars to the gaseous cooling medium (hydrogen or air). The principal heat flux takes place in this case from the bar copper via the insulation into the tooth region of the stator laminated core. The heat is dissipated to the cooling medium from the stator teeth.
The limits of the warming are prescribed in this case by the temperature sensitivity of the insulation and, moreover, by corresponding maximum temperatures which are fixed in standards by different insulation classes (ANSI, IEC).
Because of the relatively large temperature difference between the stator copper conductors embedded in the insulation and the outer layer of the insulation, the maximum power of an indirectly gas-cooled turbine-driven generator is limited by the classical cooling principle.
For this reason there has been no lack of proposals for improving the cooling, it being the case that these efforts have concentrated, on the one hand, on intensifying the cooling in the central machine region and, on the other hand, on a fundamental reconfiguration of the cooling circuit, specifically on a transition from conventional pressure cooling to reverse flow cooling, as is described, for example, in the CIGRE Report "Development of Large Air Cooled Generators for Gas Turbines and Combined Cycles", in particular FIG. 1 on page 11-201, 1992 Session 30 Aug.-5 Sep. 1992.
In general, by comparison with pressure cooling, reverse flow cooling offers the advantage that the air leaving the coolers can be fed directly to the cooling ducts in the stator and that the temperature increase caused by the machine fan is eliminated. It could be regarded as disadvantageous that the guidance of the cooling gas in the machine becomes more complicated overall because, in particular in the overhang space, it is necessary to provide additional built-in components, and supplying the rotor with cooling gas also becomes more difficult.
A further problem is the arrangement of the coolers in machines with relatively high specific outputs. Accommodating the coolers in the housing is scarcely possible without increasing the housing diameter.